1. Field of the Invention
The present invention relates to the fields of computer graphics, and desktop publishing, and to production of animation and cartoons or the like. More particularly, it relates to a method and program for generating a cartoonish two-dimensional image from data regarding a three-dimensional object within a virtual space.
2. Description of the Prior Art
With improvements in the performance and reduction of the cost of computers and the general availability of associated software, computer graphics technologies have come to be used directly in the production of animations, and the production of cartoons and comics and the like. In the field of computer graphics in the past, three-dimensional objects have been rendered in a virtual space. It is advantageous for the production engineers to utilize the ability to create an object in a virtual space and to verify the finished object on a computer while moving it in various ways and changing its orientation is a great advantage, for shortening actual production process, even in creating a still image that is to be expressed in two-dimensionally at the final stage, such as cartoon. Also, in the field of computer game, computer graphics images are provided to a user as a screen display that is finally shown in the two-dimensional form.
In such fields, in addition to the desire to represent a two-dimensional image viewed by a user in a more natural and precise form that looks like the actual object, there is a desire to make use of a familiar method of representation that is commonly used in the conventional production of cartoons and animations (hereinafter referred to as cartoonish picturization) as a method of representing images.
The term cartoonish picturization as used herein encompasses methods such as the method representing a border between a plurality of three-dimensional surfaces using a “main line” or an emphasized line with a certain thickness, the method of emphasizing the main line representing the border between a background image and a three-dimensional object as an outline, and the method of using “screen tone” to represent a shadow formed in relation to the position of a light source and the existence of a three-dimensional curved surface or three-dimensional object, or representing the brightness of that part using a specific dark coloring. In the past, the artist would employ combinations of such methods, either manually or by DTP techniques.
However, there has been no disclosure of a method for automatically generating such cartoonish picturization from three-dimensional object data.
In the Japanese Patent Provisional Publication 7-129762, there has already been a disclosure of a method for extracting and reconstructing an outline or a shading from an image representing a two-dimensional representation. Using this method, an outline or shading is generated based on information that is found only in the two-dimensional image. For this reason, it is not possible to achieve good representation of main lines by application of this method to computer graphics. For example, it is not possible to use main lines to represent borders between two surfaces placed parallel each other and having different distances (depths) from the view point but the same brightness. This is because a border between those two surfaces shows almost no contrast in a photograph of an actual object or in a computer graphics image. Even in a conventional method without using a computer, it was possible to achieve a cartoonish picturization by the artist adding a main line as required at such borders, however, it is not possible to make such representations using a computer.
In the Japanese Patent Provisional Publication 2001-319244, there is an attempt to add a shading tone image to a three-dimensional object. According to this method, it is possible to use shading tone representation for objects. However, since main lines and outlines are not generated from a three-dimensional object, it is not possible by the mere use of shading tone to obtain cartoonish picturization.